Manufacturing of an integrated circuit (IC) has been largely driven by the need to increase the density of the integrated circuit formed in a semiconductor device. This is typically accomplished by implementing more aggressive design rules to allow larger density of IC device to be formed. Nonetheless, the increased density of the IC devices, such as transistors, has also increased the complexity of processing semiconductor devices with the decreased feature sizes.
Transistors in the integrated circuit have typically been formed with a silicon gate oxide and polysilicon gate electrode. There has been a desire to replace the silicon gate oxide and polysilicon gate electrode with a high-k gate dielectric and metal gate electrode to improve device performance as feature sizes continue to decrease. Transistor having metal gate electrode, especially in conjunction with high dielectric constant (high-k) dielectric, may address issues such as polysilicon depletion and gate leakage associated with silicon gate oxide. In addition, metal gate transistors exhibit a lower resistivity than doped polysilicon. In the manufacturing process of the metal gate transistors, further improvements are constantly necessary to satisfy the performance requirement in the scaling down process.